Additive manufacturing or rapid prototyping methods for producing objects comprise layer-by-layer solidification of a material, such as a metal powder material, using a high energy beam, such as a laser beam or electron beam. A powder layer is deposited on a powder bed in a build chamber and a laser beam is scanned across portions of the powder layer that correspond to a cross-section of the object being constructed. The laser beam melts or sinters the powder to form a solidified layer. After selective solidification of a layer, the powder bed is lowered by a thickness of the newly solidified layer and a further layer of powder is spread over the surface and solidified, as required.
During the melting or sintering process, debris (e.g. condensate, unsolidified particles of powder etc) is produced within the build chamber. It is known to introduce a gas flow through the build chamber in an attempt to remove debris from the chamber in the gas flow. For example, the M280 model of machine produced by EOS GmbH, Munich, Germany comprises a series of gas outlet nozzles located in the build chamber to the rear of the powder bed that pass a flow of gas to a series of exhaust vents that are located in the build chamber at the front of the powder bed. In this manner, a planar layer of gas flow is created at the surface of the powder bed. A similar arrangement is provided in Renishaw's AM250 and AM125 machines, wherein apertures in the build chamber either side of a powder bed provide substantially planar gas flow across the powder bed.
A problem with the gas flow generated by such arrangements is that the gas flow may not be sufficient to carry all debris to the exhaust vents and some of the debris may be blown onto powder that has yet to be scanned. This can result in the object being built inaccurately. For example, the debris blown onto the powder can solidify to the object being built resulting in a solid projection from the bed that contacts a wiper blade when the wiper spreads the next layer of powder. This projection can cause damage to the wiper blade, which in turn, can result in subsequent layers of powder having a ridge at a location corresponding to the area of the wiper blade that is damaged. These ridges are created in each layer for the rest of the build, affecting the accuracy with which the object is built.
Furthermore, it is known to vary the direction in which the laser is scanned for different layers and/or for different portions of a layer, for example, see US2008/0241392 and US2005/0142024. It may be desirable to scan the laser in a particular direction based on gas flow direction, for example as set out in patent applications U.S. 61/791,636 and U.S. 61/774,215, which are incorporated herein by reference. However, with the apparatus described above, it may be necessary to compromise between optimum scan direction based on gas flow direction and the desire to change the scan direction for different layers and/or different portions of a layer.
U.S. Pat. No. 6,215,093 describes apparatus wherein a nozzle for providing a protective gas stream travels together with the laser beam.